This application is related to Japanese applications Nos. 2000-267086 and 2001-126110, filed on Sep. 4, 2000 and Apr. 24, 2001 whose priorities are claimed under 35 USC xc2xa7 119, the disclosures of which are incorporated by reference in their entirety.
1. Field of the Invention
The present invention relates to an improvement in output of a light emitting diode (LED) comprising a multiple quantum well (MQW) layer and a reflecting layer below the MQW layer. In particular, it relates to an LED comprising an MQW layer made of an AlGalnP type material and a distribution type Bragg reflection (DBR) layer as a reflecting layer.
2. Description of Related Art
A Semiconductor device utilizing an AlGaInP type semiconductor material has been employed as a visible light emitting device, since the material is capable of lattice coupling with a GaAs substrate and shows the greatest direct transitional bandgap among III-V group compound semiconductors. As a light emitting diode, the semiconductor device performs direct transitional light emission in the range of 550 to 690 nm, which exhibits a high light emitting efficiency.
The device employs, as an active layer, a multiple quantum well (MQW) layer capable of showing higher light emitting efficiency than a bulk active layer. Higher output is obtained by increasing the number of well layers comprising the MQW layer. It is considered desirable that the number of the well layers is 10 or more (see Japanese Unexamined Patent Publication No. HEI 6(1994)-112528).
FIG. 3(a) shows an example of a conventional light emitting diode made of AlGaInP. In FIG. 3(a), reference numeral 11 signifies an n-type GaAs substrate, on which an n-type AlGaInP clad layer 13, a MQW layer (active layer) 34 of AlGaInP type material, a p-type AlGaInP clad layer 15, a p-type GaInP electric current diffusion layer 16 are formed. Further, electrodes 17 and 18 are formed on a p-side and an n-side, respectively to complete the light emitting diode. As shown in FIG. 3(b), the MQW active layer is comprised of ten or more AlGaInP type well layers 34a and ten or more barrier layers 34b that are alternately stacked.
To improve output of the conventional LED, it may be a possible manner to form a reflecting layer below the active layer to reflect and take out light emitted toward the substrate. According to the construction, light which has been absorbed in the substrate can be extracted. Therefore, it is considered that the output will be doubled if a reflectance of the reflecting layer is 100%.
Actually, an AlGaAs type reflecting layer of a DBR structure shows the reflectance of 95% by itself. However, even if the reflecting layer is applied to the LED, improvement of the output is less than 95% because the active layer absorbs light reflected on the reflecting layer when it passes through the active layer. Where the light absorption of the active layer is remarkable, the reflecting layer becomes ineffective.
An object of the present invention is to improve output by optimizing a layered structure of the MQW active layer to suppress the absorption of the reflected light by the active layer.
According to the present invention, in an LED comprising an MQW layer and a reflecting layer below the MQW layer, higher output is obtained by optimizing the structure of the MQW layer to suppress the absorption of the reflected light.
Thus, the present invention provides a light emitting diode comprising an MQW layer as an active layer and a reflecting layer below the active layer, wherein the number of well layers in the MQW layer is determined such that the MQW layer shows an external quantum efficiency higher than that of an MQW layer including a single well layer.
The present invention further provides a light emitting diode comprising an MQW layer as an active layer and a reflecting layer below the active layer, wherein a total thickness of well layers in the MQW layer is determined such that the MQW layer shows an external quantum efficiency to the maximum.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.